1. Field of the Invention
This invention relates generally to the measurement of electrical conductivity of a liquid, and particularly, it concerns an improved conductivity probe for use with electrical conductivity measuring instruments.
2. Description of the Prior Art
The determination of the conductivity of an electrolyte such as an aqueous liquid has been an elusive task for over one hundred years. A multitude of techniques and apparatuses were tested before relatively satisfactory systems were found for this purpose. The greatest error in prior attempts to measure electrolytic conductivity were caused by the polarization of electrodes which were employed for potential sensing and current source purposes. Attempts to avoid the problem with the polarization of electrodes resulted in two separate electrical systems being developed for measurement of conductivity.
The polarization of electrodes can be eliminated by using alternating current of relatively high frequency and stability combined with very special electrodes coated with platinum black. These electrodes expose a large surface to an electrolyte so as to reduce the surface density of ions being deposited and thereby reducing the polarization effects.
The use of d.c. measurement system for determining conductivity has several advantages, the principal advantage being in avoiding a precisely settable and stable high frequency oscillator for generating the alternating currents of the preceding a.c. measurement system. Many attempts were made to employ d.c. currents for the measurement of conductivity of aqueous liquids and other electrolytes. Nearly one hundred years ago, a method was developed in which the polarization effects of electrodes could be reduced to manageable proportions for producing reliable conductivity measurements. In such a method, a constant current is passed through the electrolyte and the drop in potential between two points in the system is measured by secondary electrodes connected to an electrometer.
The use of such a plurality of electrodes implements a novel technique where current flow between a first set of electrodes creates a polarization potential between an additional set of potential monitoring electrodes. In such arrangement, the change in current can be correlated to the change of polarization potential at the reference electrodes. Thus, such measurements can be correlated directly to the electromotive force which opposes the flow of current through the electrolyte. The problem of polarization at the electrodes can be overcome by applying a small finite current flow and measuring the induced polarization potential change. Pursuant to Ohm's Law, the smallest polarization potential change is induced by a corresponding current flow. The original polarization potential at the measurement electrodes is eliminated in a linear system. Now, the measurement system follows Ohm's Law, the polarization potential about the measurement electrodes and the current flow inducing same are directly proportional and simultaneously approach zero magnitudes.
In many of the known direct current measurement systems for determining conductivity of electrolytes, the measurement is made in what is termed an "open" cell. The electrodes are immersed within a liquid, but the system is exposed to atmospheric and other external forces, which forces undesirably effect the current flow between the electrodes. Thus, movement of the electrodes relative to the container, the placement into the solution of some foreign material such as an air bubble, and stirring or like mechanical displacement, severely affected the accuracy of these d.c. measurements with electrodes in an open cell.
Complicated conductivity probe design may be employed with "closed" cells in which the conduction path for the electric current between a pair of electrodes is confined to a fixed volume of liquid within the completely enclosed nonconductive structure to avoid external interference forces. However, such probe designs are substantially affected by liquids carrying solids which deposited within the confined areas of the probe. As the solids and other deleterious materials become entrapped in these complex probes, the calibration accuracy of the probes begin to vary thereby affecting the conductivity measurement. Repeated standardizations and recalibrations or cleaning of the probe are required to maintain accurate measurements.
The present invention is a conductivity measurement probe of the closed cell type, but one which has a relatively simple design for confining the current flux within a channel and with such electrode configuration that the calibration constant of the probe is substantially determined only by the physical dimensions of the flux channel and electrode spacing and not by the surface area, size, shape or corrosion and scaling of the electrodes.